Roughly 95% of solar cells are manufactured using silicon as a substrate. The need to produce more electric power per kilogram of silicon in periods of poly silicon shortage has lead to rigid thickness reductions for single crystal and polycrystalline silicon wafers. Silicon wafers used in solar cell manufacture are often very thin, e.g., only about 250 microns thick. The mechanical strength of single crystal and polycrystalline silicon wafers and cells is strongly dependent on the length and the position of micro cracks in the silicon wafer material. Micro cracks increase the breakage risk over the whole value chain from the wafer to the finished module, because the wafer or cells is exposed to tensile stress during handling and processing. The use of larger and thinner crystalline silicon wafer increases the risk of yield loss.
About 5% to 10% of solar cell wafers break during manufacturing. Wafer breakage during processing is a very high cost issue. This is particularly true when wafers fail during one of the print steps, general resulting in several minutes of downtime while the operator cleans up the scattered parts and the wet paste. This is also a source of potential contamination. It is believed that wafers frequently fail at the print steps because they come into the process already cracked and the crack then fails when it is stressed during the process step. Wafer cracks can also cause electrical failure during cell or module testing.
Some techniques have been used for detection of micro-cracks of silicon wafer. Laser-based ultrasound technique is based on using a short laser pulse directed at the wafer to cause a sudden rise in temperature of the wafer material. The temperature rise initiates a sudden but minor expansion of the silicon. The acoustic energy released from the expansion can be used to distinguish between elastic or plastic expansion. The strain energy emitted from the cracks will produce acoustic waves having frequencies characteristic of plastic deformation.
Another technique is Scanning Acoustic Microscopy (SAM). In SAM, a focused acoustic beam is scanned over the front and back surface of the wafers. The sound pulses are transmitted through the wafer and the reflection from the wafer interface is monitored. The ultrasonic pulses are generated by high-frequency piezoelectric transducer. Electrical pulse from high voltage transmitter is converted to mechanical energy. This activation causes the transducer to vibrate at a specific frequency causing ultrasonic pulses to be transmitted from the transducer. These pulses travel through the material at the material velocity and are reflected at the interfaces of the material it strikes. The ultrasonic energy does not travel well through air so the wafers have to be placed in a coupling medium (e.g., a deionized water bath). However, each SAM wafer measurement may take as long as 20 minutes for sample set up and data collection. This is too slow for an inline production process.
There are optical techniques available to detect large and obvious cracks on surface of wafers after the wafers enter the manufacturing line. Examples of such systems include those made by ICOS Vision Systems of Heverlee, Belgium. These systems are based on cameras that collect an image of a solar cell wafer and analyze the image to detect a variety of defects including large surface cracks. Unfortunately, cracks are not always visible to cameras for several reasons. In some cases, the cracks might be sub surface cracks that are not visible to the camera. In other cases the cracks are generated during processing steps such as thermal processes.
To the inventors' best knowledge, there is no commercial inspection technique that images cross-sections of the wafers and screens out cracked wafers or wafers that are liable to crack before they enter the manufacturing line.
It is within this context that embodiments of the present invention arise.